The selective detection of individual respiratory droplets in air

Malok, Matjaž (author), Kavšek, Darko (author), Remškar, Maja (author)

Preventing the spread of airborne diseases in crowded indoor spaces is a global challenge. Infected individuals release virus-laden respiratory droplets (RDs) that can remain suspended in air and infectious for hours. Current monitoring methods cannot distinguish these droplets from airborne particulate matter (PM) in a real time. Here, we present a capacitive sensor that selectively detects and counts the individual droplets in indoor spaces, regardless the presence of PM. The device exploits the dielectric constant (ε) of water (78.2) to differentiate the droplets from solid PM particles (ε < 15). In a nonventilated conference-room study, RDs concentrations (40–330 RDs/L) were found to be correlated with human occupancy, but not with PM$_{2.5}$ levels. The developed technology enables a real-time monitoring of number concentration of RDs, which represent a potential health risk when they carry viral or bacterial infections. The detected increase in RD concentration can serve as a trigger for data-driven ventilation and infection-prevention measures, providing an effective tool for mitigating the spread of respiratory diseases in hospitals, schools and other public spaces.

• Language: English
• Keywords: respiratory droplets, capacitive sensing, indor air monitoring, infection control, airborne transmission, atmospheric chemistry, electrical properties, infectious diseases, liquids, nanoparticles
• Work type: Article
• Typology: 1.01 - Original Scientific Article
• Organization: FMF - Faculty of Mathematics and Physics
• Publication status: Published
• Publication version: Version of Record
• Year: 2026
• Number of pages: Str. 55–62
• Numbering: Vol. 11, iss. 1
• PID: 20.500.12556/RUL-178331
• UDC: 53
• ISSN on article: 2379-3694
• DOI: 10.1021/acssensors.5c02057
• COBISS.SI-ID: 263994627
• Publication date in RUL: 23.01.2026

Record is a part of a journal

• Title: ACS sensors
• Publisher: American Chemical Society
• ISSN: 2379-3694
• COBISS.SI-ID: 526244889

Licences

• License: CC BY 4.0, Creative Commons Attribution 4.0 International
• Link: http://creativecommons.org/licenses/by/4.0/
• Description: This is the standard Creative Commons license that gives others maximum freedom to do what they want with the work as long as they credit the author.

Secondary language

• Language: Slovenian
• Keywords: kapljice izdihanega zraka, zrak v zaprtih prostorih, monitoring zraka, nadzor okužb, aerogene okužbe, detektorji